The inkjet recording techniques have been drawing attention as suitable recording techniques for office use. That is because, firstly, inkjet recording can be performed at a high speed. Secondly, inkjet recording can be performed on a sheet of plain paper without having to perform any specific image fixing process. Thirdly, the noise generated during inkjet recording is sufficiently small so that it can be ignored. A variety of inkjet recording techniques have been disclosed and some have already been commercialized for actual use. An inkjet recording technique makes use of an inkjet head that includes an ink liquid room and a series of nozzles connected to the ink liquid room. Depending on image data, certain pressure is applied to the ink in the ink liquid room. Consequently, small ink dots are discharged through the nozzles on a recording member such as paper or a film. The discharged ink dots get attached to the recording member whereby an image is formed on the recording member. Depending on the configuration of the inkjet head, an inkjet printer can be classified as a serial inkjet printer or a line inkjet printer. A serial inkjet printer forms an image by moving the inkjet head across the width direction of the paper sheet (main-scanning) and advancing the paper sheet after completion of one or more scanning to form the subsequent recording line. On the other hand, in a line inkjet printer, the nozzles are arranged over substantially the entire region along the width direction of the paper sheet. In that case, the inkjet head does not move along the width direction. Rather, an image is formed while advancing the paper sheet beneath the inkjet head. Since a line inkjet printer can form a single recording line in the width direction at once, a high-speed recording can be obtained. However, because of the nozzle arrangement over substantially the entire region along the width direction of the paper sheet, the size of the inkjet head increases. That causes an increase in the size of the line inkjet printer. Moreover, to perform high-resolution recording in a line inkjet printer, the nozzles need to be arranged in a precise manner in the inkjet head. That leads to an increase in the manufacturing cost of the inkjet head. In comparison, a serial inkjet printer can form an image with a relatively smaller inkjet head thereby enabling achieving reduction in the manufacturing cost. That is why, at present, a variety of commercialized serial inkjet printers are available in the market.
Typically, an inkjet recording apparatus outputs an output image either in the three subtractive primary color components of cyan (C), magenta (M), and yellow (Y) or in four color components by generating black (K) from the three subtractive primary color components. To output such an output image, the three color components of red (R), green (G), and blue (B) of input image data are first converted into CMY image data or CMYK image data, which is multi-level image data. Then, the multi-level image data is converted into two-valued image data by pseudo-halftone processing and the output image is formed with the use of a recording material of each color component.
To convert the multi-level image data into two-valued image data, an error diffusion technique can be employed as described in “An adaptive algorithm for spatial gray scale” by R. Floyd et al., SID International Symposium Digest of Technical Papers, vol 4.3, 1975, pp. 36-37. In that error diffusion technique, quantization error generated in a particular pixel is diffused over a plurality of subsequent pixels to express gradation in a pseudo manner. Moreover, while performing pseudo-halftone processing on CMYK multi-level image data, error diffusion processing according to the abovementioned error diffusion technique is performed independently for each color component of cyan (C), magenta (M), yellow (Y), and black (K). As a result, the generated two-valued image of each color component has superior visual quality. However, if two or more color components are synthesized, then the output is not necessarily of acceptable quality from a visual perspective.
FIG. 19 is a schematic diagram depicting a two-valued image 601 of cyan pixels, a two-valued image 602 of magenta pixels, and a two-valued image 603 generated by synthesizing the cyan pixels and the magenta pixels. Each of the two-valued images 601 and 602 is formed by performing the error diffusion processing on image data having a uniform pixel value. Thus, the pixels in each of the two-valued images 601 and 602 are evenly spaced apart from each other. That enhances the visual quality of the two-valued images 601 and 602. In comparison, the two-valued image 603 is generated by synthesizing the cyan pixels and the magenta pixels in the two-valued images 601 and 602, respectively. However, since the pixel positions of the cyan pixels and the magenta pixels have no correlation, the cyan pixels and the magenta pixels are not evenly spaced apart in the two-valued image 603. Moreover, overlapping of pixels (cyan pixels and magenta pixels) can be observed even in the regions where only few pixels are present. Thus, it cannot be said that the two-valued image 603 is good in visual quality.
To guard against such a problem, Japanese Patent Application Laid-open No. 2007-6391 discloses a technique of performing error diffusion processing on the total pixel value calculated by adding the individual pixel values and then optimally arranging the dots in, for example, the descending order of dot diameter or dot density).
However, due to dot gain or developing characteristics in electrophotography, the pixel values and the dot count/the quantity of recording material do not necessarily have a correlation. Thus, in the technique disclosed in Japanese Patent Application Laid-open No. 2007-6391, there are times when a dot arrangement of an optimal dot count/optimal quantity of recording material is not achieved.
The present invention has been made to solve the above problems in the conventional technology and it is an object of the present invention to provide a technology that enables achieving optimal diffusion in dot arrangement of all color components in multi-level image data of two or more colors and preventing color overlapping for each color component.